Quantitative and qualitative changes in androgen pathways during physiological aging will be studied. The production of testosterone, 7alpha-hydroxy-testerone and estradiol-17beta will be quantified both in vivo and in vitro in young and aged male rats. The cellular mechanism involved in the loss of tissue responsivity to gonadotropins is also studied. The ability of interstitial tissue and seminiferous tubules to bind iodinated hCG and FSH, respectively, will be studied relative to age changes. Stimulatory effects of hCG and FSH on steroidogenesis will also be determined to correlate with the changes in number of gonadotropin binding sites.